The reception and transmission of electronic signals is generally accomplished using some type of antenna structure. Ideally, a single compact antenna would be able to adequately handle a wide bandwidth. However, there are significant limitations to such an ideal antenna. With respect to monopole and dipole antennas, there have been various efforts to extend the bandwidth and varied designs to improve performance over a certain bandwidth. However, these antennas have a limited operational bandwidth, as the bandwidth is related to the physical dimensions of the length of the antenna as well as other factors such as the frequency of interest, the variation of impedance, and the radiation pattern.
Linear antennas such as dipoles and monopoles have a well-known bandwidth limitation in the order of 3:1. Various schemes have been used to extend the bandwidth consisting of a series of tuned traps and resistors. However, the traps give multi frequency response to linear antennas, but relative constant characteristics have been difficult to obtain. The resistors make the antennas a traveling wave structure, but pattern performance is still limited to about 3:1.
There are several techniques to obtain broadband antenna operation. One scheme uses low loss resonant circuits inserted into the linear antenna at strategic points. For discussion purposes it will be assumed that the monopole extends over an infinite ground plane. A λ/4 wavelength long antenna has a radiation pattern that has a null on the axis of the monopole and peak energy on the ground plane. Extending the length or raising the frequency to an equivalent length of 5/8λ increases the gain on the horizon (ground plane) and starts to form a secondary lobe.
When the antenna reaches one wavelength, the beam peak has lifted off of the horizon to about 45 degrees. Now if a parallel resonant circuit is placed along the wire at the λ/4 length, ideally this will maintain the 4×f1 currents at a length less than λ/4. This occurs because the parallel resonant circuit presents high impedance to the current at 4×f1, and disconnects the remaining length of the antenna for the resonant frequency of the parallel resonant circuit (trap) but on the high side of resonance the net reactance is low thus reconnecting the extremity of the antenna and preventing operation above 4×f1. Current schemes have attempted to place a large number of traps in a log periodic fashion along the length of the antenna with somewhat limited success.
FIG. 3a and FIG. 3b illustrate two terminal filters used in state of the art designs. These present type of filter designs have several short comings as described herein. A capacitor and an inductor can be coupled in series, such as shown in FIG. 3a, or coupled in parallel as shown in FIG. 3b to achieve certain filtering characteristics or design criteria. One such short coming of two terminal filters is that there are only two terminals provided for any filtering action.
There have been other attempts to achieve broadband operation by placing a resistive element about λ/4 from the far end of an antenna. This technique tends to improve the VSWR and operate over a substantially wider portion of the frequency range than did multiple traps. The beam lifts off of the ground plane above the frequency associated with a monopole length of about 0.8λ.
Another concept inserts resistors at even increments along the antenna. This concept showed that an antenna could be developed for extremely wide bandwidth by isolating the extremities and allows the use of elements at multiple frequencies. But, this concept has low efficiency because of the liberal use of resistors. Also, the use of resistors would in general limit the use to relatively low power operation.
What is needed, therefore, are techniques for providing broadband coverage without the aforementioned problems. There should be a broadband antenna system that enhances existing designs for manufacturability and ease of implementation, but that expands the bandwidth coverage.